Human tooth enamel naturally undergoes a process of demineralization. Exposure of enamel to saliva and food slowly leaches minerals from teeth and eventually leads to increased susceptibility to decay. This process of demineralization results in incipient caries which are typically very small defects in the enamel surface that are thus far usually left untreated. Carious dentin demineralization also may occur in patients that have exposed regions of dentin resulting from decay below the cementum-enamel junction. Accordingly, there has been much work associated with slowing this natural process of demineralization including the application of fluoride and other topical treatments.
For example, U.S. Pat. No. 5,427,768 discloses calcium phosphate solutions which are supersaturated with respect to calcium phosphate solids and carbon dioxide. The solutions deposit calcium phosphate compounds with or without fluoride on and in the tooth weaknesses such as dental caries, exposed root, or dentin. U.S. Pat. Nos. 5,268,167 and 5,037,639 disclose the use of amorphous calcium compounds such as amorphous calcium phosphate, amorphous calcium phosphate fluoride and amorphous calcium carbonate phosphate for use in remineralizing teeth. These amorphous compounds, when applied to dental tissue prevent and/or repair dental weaknesses. The disadvantages of these methods include (1) a low pH necessary for the application which can be an irritant, (2) rapid reaction results in a very short term effect, (3) since these methods use solutions, the actual reactions are difficult to control from patient to patient, and (4) since the reactions are rapid and of short duration, the procedure must be repeated to maintain the effect. Also, both methods require maintaining at least one solution with pressurized CO.sub.2 prior to mixing delivery which makes the method difficult to incorporate into an over-the-counter procedure.
Demineralization eventually leads to cavitation of enamel coating such that there is exposure of the underlying tooth structure. Typically, this type of decay is treated by drilling out the decayed region and inserting a semi-permanent filling material. However, there is a need for a less invasive means of arresting and reversing decay.
Prophylactic pit and fissure sealants have become widely used in preventing decay in areas that are particularly at risk for decay. These sealants have included polymer or other cements that require a dry application and the use of a fixing agent. These sealants are temporary and do not provide for an optimal seal.
Liners and bases are materials that are used to treat newly exposed tooth surfaces such as those surfaces exposed by drilling. After a cavity is prepared, it is common practice to apply a liner or base before filling the cavity with a filling material. A liner is a thin coating of material and a base is a thicker coating. Liner and base materials are designed to decrease permeability of dentin at the tooth material interface and protect against microleakage around and through the fill material and to seal dentin tubules. Earlier liners or "cavity varnishes" include materials such as organic "gums" dissolved in organic solvents. Upon evaporation of the organic solvent, the gum is left behind. Disadvantages associated with these organic gums are well documented and include leaky junctions, lack of adherence, acid vulnerability, etc. Another method of lining is disclosed in U.S. Pat. No. 4,538,990 which describes applying a 1 to 30% w/v neutral oxalate salt solution, such as dipotassium oxalate to the smear layer and then applying a 0.5 to 3% w/v of an acidic oxalate salt solution such as monopotassium monohydrogen oxalate to the layer. Research has shown poor seal occlusion of the tubules with this method.
U.S. Pat. No. 5,296,026 discloses glass phosphate cement compositions and methods for their use as surgical implant materials to fill cavities in bone and canals in teeth. The cement compositions include P.sub.2 O.sub.5, CaO, SrO and Na.sub.2 O in combination with an aqueous liquid with or without therapeutic agents. Mixing the powder and liquid results in a hardening reactions. When the cement is implanted into hard tissue, it serves as a filler/graft material and along with the release of leachable constituents it can assist in the healing and maintenance of healthy bone.
Various bioactive and biocompatible glasses have been developed as bone replacement materials. Studies have shown that these glasses will induce or aid osteogenesis in a physiologic systems. Hench et al, J. Biomed. Mater. Res. 5:117-141 (1971). The bond developed between the bone and the glass has been demonstrated to be extremely strong and stable. Piotrowski et al., J. Biomed. Mater. Res. 9:47-61(1975). Toxicology evaluation of the glasses has shown no toxic effects in bone or soft tissue in numerous in vitro and in vivo models. Wilson et al., J. Biomed. Mater. Res. 805-817 (1981). The glass has been reported to be bacteriostatic or bacteriocidal most likely related to the change in pH induced by the dissolution of the ions from the surface of the glass and lack of bacterial adherence to the glass surface. Stoor et al, Bioceramics Vol. 8 p. 253-258 Wilson et al (1995).
The bonding of the glass to bone begins with the exposure of the glass to aqueous solutions. Na.sup.+ in the glass exchanges with H+ from the body fluids causing the pH to increase. Ca and P migrate from the glass forming a Ca--P rich surface layer. Underlying this Ca--P rich is a layer which becomes increasingly silica rich due to the loss of Na, Ca and P ions (U.S. Pat. No. 4,851,046).
The behavior of the bioactive glass as solid implants in a dental application was reported by Stanley et al., Journal of Prostetic Dentistry, Vol. 58, pp. 607-613 (1987). Replicate tooth forms were fabricated and implanted into extracted incisor sockets of adult baboons. Successful attachment of the implants to surrounding bone was seen after histologic examination at six months. Clinical application of this technique is presently available for human use. Endosseous Ridge Maintenance Implant ERMI.RTM.. Particulate bioactive glass has been used for periodontal osseous defect repair (U.S. Pat. No. 4,851,046) utilizing a size range of 90-710 .mu.m and a compositional range described in the following chart.
______________________________________ Component Weight Percentage ______________________________________ SiO.sub.2 40-55 CaO 10-30 Na.sub.2 O 10-35 P.sub.2 O.sub.5 2-8 CaF.sub.2 0-25 B.sub.2 O.sub.3 0-10 ______________________________________
Previously described data has shown that 60% silica is beyond the limit of bioactive melt derived glasses. Okasuki et al. Nippon Seramikbusu Kyokai Gakijutsu Konbuski, Vol. 99, pp. 1-6 (1991).
The 90-710 .mu.m size range was determined to be the most effective for periodontal applications when in direct contact with bone. However, size ranges smaller than 90 .mu.m were ineffective due to their high rate of reactivity and rapid resorption at the bony site. Moreover, size ranges smaller than 90 .mu.m were determined to be ineffective in soft tissue sites also due to the presumption that the smaller particles were removed by macrophages (see U.S. Pat. No. 4,851,046). A size range of less than 200 .mu.m was also found to be ineffective in certain bone defects (see U.S. Pat. No. 5,204,106) due to the high rate of reactivity.
U.S. Pat. No. 4,239,113 ("the '113 patent") also describes the use of a bone cement. The '113 patent only discloses bioactive glass ceramic powder having a particle size of 10-200 microns. Moreover, the '113 patent also requires the use of methylmethacrylate (co)polymers and vitreous mineral fibers.
None of the foregoing methods or compositions provide for the combined advantages of both easy application and adherence to tooth structure including penetration into very small tooth structure defects and the opportunity for continued chemical and physical interaction with tooth structure after application.
Accordingly, it is an object of the present invention to provide a composition capable of chemical and physical interaction with tooth structure that is easily applied and readily adherent to tooth structure.
It is a further object of the invention to provide a method of using such a bioactive glass composition to treat a variety of dental and other conditions.